Magnetic sensors are well known in the art. Referring to FIG. 1A there shown a cross sectional view of a magnetic switch 10 or a reed switch 10 of the prior art. The magnetic switch 10 has three terminals: a first terminal 12, a second terminal 14, and third terminal 16. A magnetic attracting member 20 has one end attached to the first terminal 12. The second end of the magnetic attracting member 20 can make electrical contact with either the second terminal 14 or the third terminal 16. In operation, one of the terminals 14 and 16 is a “dummy” terminal, in that it does not have any electrical connection to the external. The other terminal connects to an alarm switch (not shown). Assume for the moment that the second terminal 14 is a “dummy” terminal and that the third terminal 16 is connected to the alarm switch.
In operation, the magnetic switch 10 is typically placed on a door frame with electrical leads connected to the first terminal 12 and the third terminal 16, as shown in FIG. 1B. A magnet 30 is attached to a door. When the door is “closed” the magnet 30 is sufficiently close to attract the magnetic attracting member 20 to pull it to the closed position thereby making electrical contact between the first terminal 12 and the third terminal 16. In this condition, the alarm switch (not shown) is then activated; in that an electrical connection is established sending an electrical signal through the connection of the third terminal 16, through the magnetic attracting member 20, through the first terminal 12. When the door is opened or disturbed, the magnet 30 is moved. When the magnet 30 is moved sufficiently far away, the magnetic attracting member 20 reverts back to its default position which is to make contact between the first terminal 12 and the second terminal 14. In this condition, the circuit between the first terminal 12 and the third terminal 16 is broken. This can then be detected by the alarm panel and an alarm can then be activated.
The magnetic sensor 10 of the prior art can be tampered or otherwise circumvented by a thief or other evil doer. As can be seen in FIG. 1B, there is a gap 32 that exists between the magnet 30 and the sensor 10. Before the magnetic sensor 10 is armed, e.g. during day time, and while the door is open, a thief could place a very thin strip of magnet immediately adjacent to the housing of the magnetic sensor 10. This supplemental magnet, would have sufficient magnetic strength to attract the magnetic attracting member 20 so that it closes the contact to electrically connect the first terminal 12 to the third terminal 16 at a time when the alarm is not activated. When it is desired to arm the magnetic sensor 10, with the door closed, the magnet 30 is moved so that it is adjacent to the magnetic sensor 10. However, since the supplemental magnet is already in contact with the magnetic sensor 10, the presence of the magnet 30 further attracts the magnetic attracting member 20 and continue to close the circuit between the first terminal 12 and the third terminal 16. In this condition, when a thief returns at night time, the thief can overcome the system by moving the door containing the magnet 30 without tripping the alarm. The supplemental magnet resting against the housing of the magnetic sensor 10 would continue to keep the magnetic attracting member 20 engaged in electrical contact between the first terminal 12 and the third terminal 16 thereby preventing the alarm signal from being activated.
Accordingly, there is a need to overcome the deficiencies of the magnetic sensor 10 of the prior art.